Implantable medical devices are available for monitoring and treating patients with heart failure. Such devices include physiological sensors for sensing signals correlated to physiological events or conditions. For example, implantable hemodynamic monitors may monitor blood pressure and heart rate. Pacemakers or implantable cardioverter/defibrillators (ICDs) are available that monitor a cardiac EGM/ECG signal and may provide heart failure therapies such as cardiac resynchronization therapy (CRT). CRT involves pacing one or more heart chambers to improve the synchronization of the heart chamber contractions and thereby improve the heart's ability to eject blood. However, not all patients respond to CRT. Currently, a challenge remains in prospectively identifying heart failure patients that will respond positively to CRT.
Progression of heart failure leads to poor oxygenation of skeletal muscle tissue. A lack of cardiac reserve, i.e., the heart's inability to function above a basal level in response to an increased metabolic demand, such as during exercise, can lead to tissue hypoxia during exercise. Poor tissue oxygenation and lack of cardiac reserve lead to exercise intolerance in heart failure patients. Many approaches to monitoring a heart failure patient involve monitoring blood pressure, heart wall motion, ejection fraction, heart rate and other hemodynamic measures. However, a need remains for sensing devices and methods for diagnostic, prognostic and therapy management purposes in heart failure patients.